B+ voltages in older Marshalls are the subject of endless discussion. My understanding is that the voltages varied from year to year, and also between individual examples. Marshall are notorious for using whatever parts were available in a pinch in the early days.

Some older Marshalls have been reported to have very high B+ supplies. I've seen vintage amps (Marshall inspired designs) where the B+ was in excess of 600v (measured at the power amp stage).

Try re-posting the question on the Plexi Palace forum. There are several knowlegable people there who'll surely be able to give you a more definitive answer. Also see the older threads.